Cold caustic fiberboard manufacture



Aug. 24, 1965 w. G. COGGAN ET AL 3,202,569

COLD CAUSTIC FIBERBOARD MANUFACTURE Filed June 22, 1961 H AR DWOOD CHIP I HARDWOOD CH P ALKALI METAL I/4 2 Hours NuOH, KOH, LiOH and I0 200F519. mixtures to give digestion liquor about 2-|O% conc.

WORK TO GIVE DRAINAGE TIME OF 20 TO 40 SECONDS HARDWOOD PULP FILTER WASH THICKEN ACIDIC REAGENT ADDITIONAL $125321; 3TI;;':I.,, I..." pu;:,|:iyos, ziqmeniz, bleachis, gain we erproomq uqens, F URNlSH BOARD FORMING MACHINE DEVIATER OOMPRESS DRY "HOT PRESS MACHINE F? SEMI LIQFSDBOARD gdNENoTggm HARDBOARD BY 313E335? 'eoom INSULATION BOARD ATTORNEY United States Patent This invention relates to Wood fiberboard, conunonly known as fiberboard, to processes for the preparation of pulp from hardwood, and processes for the preparation of fiberboard from such pulps.

IELD OF INVENTION Fiberboard is a general term applied to sheets or boards manufactured from fibers of wood or other lignocellulosic fiber material with the primary bond derived from the arrangement of the fibers and their inherent adhesive properties. Bonding agents or other materials may be added during manufacture to increase strength, resistance to moisture, fire, insects or decay, or to improve some other property of the product. Such fiberboards' are to be distinguished from particle boards, as the former type of boards rely upon interfelting of the fibers, wh'ch interfelting produces a mat with a characteristically natural bond, while the latter boards do not partake of the interielting characteristics of the fibers.

Piberboards may be classified into several categories; for the purpose of this invention, the classification system found in -iberboard and Particle Board published. by the Food and Agriculture Organization of the United States (Rome, ltaly, 1958), is used. The fiberboard is classified therein either as a non-compressed fiberboard or as a compressed fiberboard. The non-compressed fiberboards have a density in the :order of about 25 pounds per cubic foot or less and may be further subdivided, on the basis of density, as semi-rigid insulation board and rigid insulation board. On the other hand, the compressed fiberboards have a density of over approximately 25 pounds per cubic foot, and this group may also be subdivided, on the basis of density, into semi-hardboard, hardbo d, and special dens-ified ha-rdboards.

Semi-rigid insulation board has a very low density, and it is used primarily in insulation. Production of this type of board is small at the present time. Rigid insulation board, one of the types of boards with which this inven tion is primarily concerned, has a density range in the order of about 9 /2 to 25 pounds per cubic foot. it is inrarily as material for house construction, cg, rig, interior paneling, acoustical panels, and as a b so for plaster or siding. it is also manufactured into -.ial forms for many additional uses. For example, it may be laminated into thick sheets for str c-tural decking and for cores of doors and partitions.

The stated top limit of density of rigid insulation board, which is generally accepted by the fiberboard industry, is determined by practical considerations, since it is the t .iit in density readily obtained on a Fourd'rinier, cyl'.der, or deckle box former without auxiliary hotnressing. l 'lany industrial and commercial specifications ectly control the top limit of density of a rigid nsulation board by specifying the maximum allowed coefiicient of thermal conductivity, which, in general, increases with density. The maximum coefficient is in the order of about 9.40 B.-t.u /hour/sq. ft./ F/inch of thickness. The thickness of such rigid insulation boards (unlarninatcd) ranges from approximately A inch to 1 inch, and the board sizes range from approximately 3 X 4' to 4' x 12. v

The compressed fiberboards (i.e., compressed under relatively high pressure and the simultaneous application of heat) of the semi-hardboard type (known also as interaeeasea ice mediate density or medium density fiberboard) vary in density from about 25 to about 50 pounds per cubic foot.

The hardboard type of fiberboard ranges in density from about 50 to about pounds per cubic foot. The densities of's-pe-cial densified hardboards are in excess of '75 pounds per cubic foot, but for purposes of this invention, they will be treated the same as hardboards. The semihardboards and hardboards may be formed in the same Way as rigid insulation board, but subsequent to the step of the forming of wet mats of fibers they are hot-pressed, either by the wet press technique or by the dry press technique.

It is to be understood that the processes of making the rigid insulation board may also be used in the manufacture of the semi-.hardbo-ard or the hardboard type of fiberboards; the primary difference between the processes of manufacturing rigid insulation board and the processes of manufacturing semi-hardboard and hardboard is that the insulation board technique does not utilize a hot-press treatment which is necessary in forming semi-hardboard and hardboard.

The procedure used for the formation of rigid insulation board is essentially a filtration or dewatering operation. In the usual process, a layer of fiber slurry is applied to a forarn-inous support upon which the fiber layer is partially dewatered to form a wet fiber mat, which is then further dewa'tered, roller or belt compressed, and subsequently dried to form the final board. Such board forming operations are generally carried out continuously, principally on so-called Wire (Fou'rdrinier) or cylinder (Sliver) machines, and consequently there is some similarity between the board forming operations and the operations used in making paper. However, the fibers needed for insulating board production are very significantly different, i.e., much coarser, than those used for paper manufacture. Moreover, the dewatering characteristics of board forming pulps are highly important to the success of a board forming process, because of the necessity of forming very thick felts, sometimes referred to as mats or webs. Actually, it is principally the thickness-es of the mats which are handled in board manufacture, as contrasted to paper manufacture, which necessitate the principal dinerences'in machinery, materials and processes employed.

Rigid insulation board is used today in enormous quantities for a wide variety of uses. Consequently, differences in cost of manufacture of the final product are critical factors in the making and selling of this type of product. It is necessary to provide insulating board having the highest possible stren th characteristics at a minimum of cost in order for the product to be accepted and purchased by the ultimate user. Speed of production is a vital factor in overall cost of insulating board manufacture, since the higher the speed at which the board can be continuously made, the lower the cost of the product. Hence it is desirable to make the board from a furnish which possesses a fast drainage time'so as to make possible the use of high forming speeds. Also, the fibers used in the manufacture of insulating board must be available in large quantities and at relatively low cost. The fibers should also be capable of forming a final dried mat having good strength characteristics without the use of substantial amounts of resins, gums, or other materials, as added binders. The fibers should also lend themselves to inexpensive pulp forming operations.

One factor which contributes to the cost of pulp used in the production of insulating board is the quantity and cost of chemical reagents required to form the pulp from Wood. if pulp forming procedures can be devised which use only relatively small quantities of inexpensive chemical reagents, or in which it is possible to recycle machines.

wood to form the processingliquors in order to digest very large quantithe cost of pulp production for insulation board manufacture will be decreased. These cost reductions may in turn be passed on to semi-hardboard and hardboard, since the latter two boards are derived from the basic insulating board or basic wet mat. The recycling of the "digestion liquors oifers'further possibilities for reductions in pulp costs, because the recycling reduces the require- 'ment for waste disposal of the liquors, which is a problem and a contributing factor to-the'over-all costs of the pulp. Thus, a pulpingoperation which would permit the digestion liquor to be recycled or reused many times, before it is necessary to discard it, would substantially reduce the portion of the pulp cost attributable to waste disposal. v

3 Because of these various limitations which are imposed upon the procedures and materials used in the production of rigid insulation board, e.g., raw material costs,

production speeds,'and the like, manufacture of this type of product has been restricted,-prior to the instant invention, to boards having densities within the limited range .in the order of about to about pounds per cubic 7 Thus, while the rigid insulating board manufacfoot. turers have been attempting to produce and offer to the trade, on an economical basis, products having a wider range of density than have been available heretofore,

they have been seriously restricted in that regard by these various limitations.

As noted, the upper limit of about 25 pounds per cubic foot is a practical limit of density for rigid insulating vboard, dictated by practicaluse of the board forming One significant factor which has contributed in the past to this limitation on density range has been 'the characteristic of available board forming pulps to rebound or spring back, when the wet mat is compressed after ithas been formed and before it is dried. Thus, after the wet mat is removed fromthe wire or cylinder machine and before itis dried, it is cold-compressed, usually by pressing rollers orbelts, to force further water from it and at the same time to increase its density.

However, with the prioravailable board forming pulps, there invariably has been a substantial amount of relbOIlI'id orspring-back in the cold-compressed wet mat,

so that before the mat can be dried, the thickness of the mat increases substantially over the thickness imparted to the-mat in the cold-compressing step. Therefore, if a board'forming pulp were available which would pro duce cold-compressed wet mats having significantly less -tenden'cy for this rebound or spring-back to occur therein, it would permit better control in the amount of compression put into and retained by the wet mat, and, consequently, would make it possible to obtain a wider range -of board densities of the final dried prOducts.

Having in mindthe problems and limitations of the art discussed above, a" principal object of this invention ;is the provision of new improvements in the manufao- I't'ure of wood fiberboard comprising hardwood fibers, and especially rigid insulating board,'usuable assuch, in a 'den sity range of about 10 to about 35 pounds per cubic ;foot, or further hot compressed for use in the form of semi hardboard or hardboard. Further objects include:

(1); The provision .of'a new variety of strong-fibers .for use in the manufacture of wood fiberboard which is compatible in all proportionswith ground wood and which possesses very'fast water drainage times, making possible production of fiberboard at high forming rates; 7 f (2) The provision of fiberboard-forming fibers, which f possess a higher degree of compressibility and lower degree ofspring-back when cold-compressed in'a wet mat than fibers available heretofore; i i I I (3) The provision of new methods for digesting hardpulp for use in the production of wood fiberboard; v p v H V 1a (4) The provision of new pulp formin g methods'for "ties of wood for a given amount of chemical reagent,

"sulating board in its wet or dry form.

production of wood fiberboard fibers which makes it possible to produce pulp in shorter time than with prior known pulping methods used in the formation of wood fiberboard fibers, and, in addition, which can be operated at relatively low digestion temperatures and with less problem as to disposal of waste liquors;

(5) The provision of such pulping 'methods which substantially reduce digester corrosion;

(6) The provision of a new variety of insulating board having a higher density than any insulating board known or available heretoforej' (7) The provision of new methods for forming insulation boards, which methods make possible a wider range in density of such products than prior known methods for their manufacture;

(8) The provision of new woodfiberboard forming fibers almost comparable in strength characteristics to kraft fibers, but with a drainage time more than five times as fast as kraft fibers; and

(9) The provision of newprocedures for the pulping of' hardwood to form wood-fiberboard-forming fibers at high yield ratesgwith low chemical reagent consumption GENERAL DESCRIPTION These objects are accomplished by the processes which can be readily comprehended by reference to the accompanying flowdiagram of the type suggested by N. O. Wolk, 30 J.P.O.S. 368, which outlines the steps and materials involved. I

Briefly described, the new procedures of this invention .for the formation of insulation board comprise chipping hardwood to form chips as uniform in size as possible, for example, forming chips capable of passing a 1-inch mesh screen but not a 4-inch mesh screen, impregnating the hardwood chips with a 2.5 to 10% by weight solution of alkali'rnetal hydroxide in Water at about to 200 F. under either atmospheric 'or hydrostatic pressure for suificient time to obtain thorough impregnation of the v chips, working the resulting chips, as by refining in a disc type refiner, until the Tappi drainage time thereof is between about 20 and 40 seconds, neutralizing the pulp to a pHof about 5 to 7 with sulfuric acid or other acidic reagent, forming afurnish from the pulp, with the inclusion, if desired, of additional furnish components, such as sizing materials, ground wood or other fiber, waterproofing agents, fireproofing agents, dyes, pigments or the like, then continuously creating a layer of the furnish on a foraminous ,sup'port, dewatering the layer of furnish through said support to create a web fiber mat,con1pressing the partially dewatered mat to remove further water therefrom and to increase the fiber-density, and finally drying the compressed web;

In-addition, the basic insulation board, manufactured according to the techniques briefly described herebefore, can also be used in'the manufacture of semi-hardboard or hardboard types of fiberboards, utilizing-the basic in- In the manufacture of such semi-har'dboard and hardboard, the insulating board is transferred to a hot press where it is compressed under a relatively high pressure under controlled conditions of pressure, time, and temperature of the press.

p Erqmpl es 1 More detailed understanding of the new procedures and products of this invention may be had'be reference to the following examples in which all parts and percentages are by weight unless otherwise specified.

Example I A standard hardwood, as, for example, re gum logs and slabs with the bark on them, are fed through a chipper to form chips of substantially uniform size, the major portion of which pass a standard sieve of 1-inch mesh size, but which are retained on a sieve of /4-lllCll mesh size. One hundred parts of these fresh, rough red gum chips are charged into a digestcr essel, of the type mounted on trunnions to permit rotation about a horizontal axis and equipped with a large charging port and inlet conduits for introduction of reaction liquors.

In a separate mixing tank, a 6% solution of sodium hydroxide in water is prepared and heated to a temperature of approximately 160 F. Approximately 500 parts of this heated caustic solution are then pumped from the mixing tank into the chip-filled digester, a rotary pump in a recycle pipe line is turned on, building up Within the digester a hydrostatic pressure of 150 p.s.i.g. If desired, the digester shell may be pro-steamed to heat the shed. The pressure is maintained for 30' minutes by automatic control of the pump, after which period the pump is stopped and air pressure is applied through an inlet valve and pipe to force the cooking liquor out of the digester though a suitable strainer opening into the adiacent storage tank. The digester is then uncapped and rotated on its trunnions to dump its load of digested hardwood chips to a receiving tank.

The complete cooking cycle takes only 110 minutes, including approximately 30 minutes to cap and load the digester, 16 minutes to pump in the cooking liquor, 15 minutes to pump the hydrostatic pressure up to 150 p.s.i.g., 30 minutes for cooking, 10 minutes for blow-down and minutes to uncap and dump the ci ester. The heart of the operation is the cooking time of minutes. If the process were to be continuous, the duration of the entire pr cess could be reduced approximately to the cooking cycle time. By a constant feed and constant withdrawal system, the 80 minutes required for loading, pumping in the cooking liquor, building up the pressure, blow-down, uncapping and unloading the digester may be virtually eliminated, so that the process is essentially broken down to the cooking cycle, per se.

Where a non-continuous process is used, the digester is reloaded with another batch of hardwood chips, while the recovered liquor in the storage tank is reheated and refortified with a caustic soda solution to bring the caustic soda content of the solution in the bank back to approximately 6% after about 20 parts of fresh water have been added. The fresh Water may also be obtained by the condensation of steam used in preheating the digestcr tank. Then the reheated refortified cooking liquor is pumped into the loaded capped digester, and the highhead, low-volume pressure pump is placed into operation to again pressurize the digester to 159 p.s.i.g. After 39 minutes of operation, the pump is again stopped, and the digester liquor is forced out of the digester, which is then uncapped and dumped as previously described.

The procedure of charging the digester, re-use of reaction liquor refortified with additional fresh caustic solution and fresh water is continued with removal for each tull cycle of quantities of accumulated reaction liquor absorbed by the chips and replacement with added quantities of fresh concentrated caustic soda solution and fresh water to maintain a substantially constant concentration of caustic and soluble digestion products in the reaction liquor. The cooked wood mix dumped from the digester is conveyed to a Bauertype double disk pulp refiner set with a plate clearance of 0.007 inch, and it is treated therein until the Tappi drainage time for the refined fiber measures between 2530 seconds. The resulting fiber slurry is then acted upon by vibrating Johnson screens, of perforations, to eliminate the +14 mesh 33.4 14l28 mesh u 2S+48 mesh 22.5 48'10=I) mesh 7.8 l9d+200 mesh 2.0

2t)6 mesh 19.6

A series of ten digester runs, conducted as described above, results in a consumption of caustic soda of approximately ltl% based upon the dry weight of the WOOCl chips charged to the operations, not including the caustic content of the reusable recovered cooking liquor to be employed in the mix cycle of operation. Also, in this series of digestions, it is found that the yield of fibers is apnroximateiy of the weight of the hardwood chips charged to the operations.

The pulp web from the deckeri-ng operation is then conveyed to a stock chest where it is mixed with fresh water, or re-use water, in sufiicient amount to produce an approximate 3 /293 fiber slurry. The pH of the cold slurry is then adjusted to approximately 6.7 by the addition of sulphuric acid streamwise to the stock chest. Approximately 1% of concentrated sulphuric acid, based upon the fiber weight of the this pH adjustment.

The neutralized fiber slurry is then allowed to mix in the stock chest; after mixing, it is pumped to the machine chest or" the board-forming unit. The slurry is maintained at a solids content of approximately 3 /2%. Approximately 1% of a petroleum wax emulsion (relative to the fiber solids of the slurry), containing 50% by weight of wax, and sufficient alum to bring the pH of the fiber to between 5.6 and 5.5, are uniformly intermixed in the slurry in the stock chest by suitable agitation thereof.

The resulting slurry mixture has additional water added thereto to bring the solids content down to approximately 1%, and the new slurry is pumped to the head box of an Oliver cylinder machine, which is operated at a matforming speed of approximately 29 feet per minute. (It is understood, of course, that a Fourdrinier machine may also be utilized if the later is available.) With the Silver cylinder operating under a vacuum of approximately 10 to 29 inches of mercury, preferably approximately l4- inches of mercury, the pulp pick-up on the cylinder machine is approximately 1,300 pounds per thousand square feet (dry weight of pulp), and the resultin, partially dewat Jed fiber mat removed from the cylinder machine onto the take-oft belt has a caliper of nated belts therearound, with the gaps between belts being ad'usted depending upon the ultimate solids content desired. In the instant case, the gaps are adjusted to result in mats having a caliper of approximately inch and a solids content of approximately 35 to 40%. These mats are cut into strips sixteen feet long and are conveyed to a conveyor belt for transportation to a tunnel drier, which they enter in substantially the stated condition.

The tunnel dryer is heated by steam pipes located therein to a temperature of approximately 340 F. Within the drier, the mats have the moisture removed thereslurry, is needed to attain which would'be reached by them under ambient temper-' The resulting boards ature and humidity conditions. possess the following average physical properties:

Thickness (inches) .0.500 Density (percubic foot) 30.6 Modulus of rupture (per square'inchnormal) 1,480

GB. puncture dry (inch-pounds) 304 /z inch edge nail (pounds) 218 Nailwith'drawa'l (pounds) .70 iBrinellhardnessNo; (1 inch ball- 50 pounds) 287 IWater absorption (2 hour-percent by volume) 6.4

The tests usedin obtaining data of physical properties reported. in the aboveexample and elsewhere in this specification are as follows:

.Tappi drainage time.This test is conducted using the :.standard drainage time method .for pulp for insulating .board Tappi T1002SM-51.

h Mullen strength- T make this determination, samples are made in'a Williams sheet mold using 25 grams dry .weight of pulp per sheet. The wet sheet is removed from the sheet mold and pressed against a steel plate on the face surface, and blotting paper to absorb the excess .water-is applied to. the screen surface. A pressure of 85 I pounds per square inch is applied for 30 seconds. Sampies are dried, trimmed to 9" x 11'. and weighed.

' Six bursts are taken on each sheet using the apparatus describedinASTM D-744-46 and the percent Mullen is calculated. h

- Bauer-McNett fiber 'cIassification.This determination is madeusing a Bauer-McNett classifier and the procedure of Tappi T233SM-53 for determining fiber length ofpulpby classification; The results are recorded in percentages by weight of the fibers retained'on each of the screens used in the classifier, namely, 14, 28, 48, 100

and 200 mesh.

=Modulus of rupture.This determination is made using the procedure specified in General Services Administration interim Federal specification LLL-F-00321b tor transverse strength from which the modulus of rupture is calculated.

GE. puncture test.Thisdetermination is made using the apparatus and procedure specified in ASTM D781- 441" with the exception that in place of the puncture point -.specified under heading Apparatus (b) there is used a :puncture point affixed to the end of the rod and having 'the shape of a half round ball of 2" diameter.

Edge nail test.This determination is made using the ,.'procedure specified in section 7, *Naill-Iolding Strength .of Building Materials 7 and Structures Report BMSl3- issued'February'23, 1939, by the National Bureau of Standards.

Brinell hardness.-This determination is made using a 1 size sample that will fit into an 8" gap. The sample is placed under the 1" ball of a dead weight tester and the tester dial is set to zer Weights are added, depending upon the hardness of the sample to give an indentation of 0.16 to 0.35". The weight load'is applied and the exact indentation depth is recorded after 15 seconds. The

Brinell hardness is then calculated using the following formula: a

, BHN Total Weight used in pounds 0.454

1000 1r 25.42 indentation depth in inches Water absorption test.This determination is made using the procedure specified in Federal specification Lug -003211. supra.

Interlaminar strength.This determination is made .using the procedure specified in Federal specification LLLF-0032lb for tensile strength perpendicular to the surface except that a specimen 2"x 2" 'is used in place of a 6 x 6" specimen'and the load is reported as .per square inch instead of per square foot.

, Moisture extensi0n.-This determination isrnade using v the procedure specified in Federal specification LLL-F- 003215 for linear expansion.

Example II i In a stock chest equipped withpropeller type heaters,

88 parts of the hardwood pulp of the kind initially prepared in Example I are charged into-5600 parts of fresh water. Then 64 parts of wet ground wood pulp made from mechanically ground cottonwood and willow fibers, having a drainage time of 60 seconds, are charged into the stock chest, along with 48 parts of asphalt solids in an emulsion form, the emulsion containing approximately V 15% byweight of asphalt and 1 part of a 50% wax size emulsion. Theseingredients are mixed for a short time in the'stock chest, and'then the pH of the slurry is adjusted to 4.9 by the addition of 1 /2 parts of alum.

When tested by standard procedure the drainage time of 'the slurry is found to be to seconds and the vat consistency to be approximately 1.2% to 1.3% solids.

By using the Oliver filter apparatus as described in Example l along with the same type of sheet forming operation, a continuous, partially dewatered mat of the -fiber and asphalt mixture is formed at a speed of about 40 feet per minute with a pick-up of about 1,000 pounds per thousand square feet (dry weight of pulp) and a thickness of about 1 inch. The partially dewatered mat, after passing the cold-compressing rollers, has a thickness of f approximately inch and a solids content of approximately 35 to 40%. In the drying ovens, the mat under- ;goes a-shrinkage of about 25% and after being trimmed 35 into'sheets, construction insulation boards having the following physical properties are obtained:

Thickness (inches) 0.500 .Density 26.0 Nail withdrawal (pounds) 43.9 /2 inch edge nail (pounds) s 143 Modulus of rupture (dry-per square inch) 1070 G.E. puncture (dry-inch pounds) 190 Interlaminar (per square inch) 17.3

Water absorption (percent by volume) (2 hours) 3.3

I DETAILED'DESCRIPTION The formation of wood fiberboard by the procedures r described herein can be accomplished using any type of hardwood which is available in large quantities and which -wood of any broad-leafed deciduous tree as distinguished is relatively inexpensive. By hardwood is meant the from the wood of coniferous trees. From the viewpoint lot availability and other considerations, the principal 'hardwoods recommended for use in the new. operations include gum, birch, willow, oak, hickory, maple, pecan,

cottonwood, and poplar, Coniferous woods arenot as suitablefor use in the process because their resinous contentresults in soap formation and foaming, and the resulting fibers do not possess the same desirable physical properties achieved when hardwoods are used.

A caustic alkali is an essential ingredient of the new pulpforming procedures. Sodium hydroxide is the preferred caustic to be used, but other alkali metal hydroxides, including. potassium hydroxide and lithium hy- The concentration of caustic alkali in the digestion liquor canfbevaried, but for most effective results the concenj tration shouldbemaintained between about 2.5% and about 10% by'weight. Above 10% concentration, the 7 0 .falls to an undesirably low level, i.e., below On dioxide, or mixtures of any of these can be employed.

yield of pulpon the dry weight of the charged wood chips the other hand, pulps prepared from cooks utilizing less than 2.5% caustic concentration contain a small amount of raw fiber which ultimately reduces compressibility and increases shrinkage. 1

The temperature of the caustic impregnation liquor when in contact with the wood chips may be varied, but preferably should be from about 70 to about 200 F. The time required for the formation of the hardwood pulp is inverse function of the temperature and pressure. The best pulp is made by the use of a digestion procedure for which it has been found that about one-half hours digestion at ambient temperature down to one-quarter of an hour at 230 F., other factors being the same, are satisfactory periods for completion of the single pulp batch. Most satisfactory results are obtained using a temperature within the range of 100 to 150 F.

The digestion procedure may be conducted at atmospheric pressure but it has been found that shorter times for processing a single batch of chips plus elimination of presence of raw fibers are obtained by the use of super-atmospheric pressures. Pressures of 308 pounds per square inch gage or higher may be used, and these pressures may be obtained in an suitable e.g., by the use of an impregnating and defioering screw press. However, it has been found that the most economical pressure for use in the process is about 160 to 156 pounds per square inch gage, and that this is best obtained by forcing digestion liquor into the digestion vesselunder this bydrostatic pressure using a rotary impeller pump or other satisfactory pumping equipment.

In order to obtain the maximum penetration of the digestion liquors into the Wood chips, vacuum may be applied to the chips before the caustic soda solution is introduced into the reaction vessel.

The ratio of digestion liquor to Wood chips in the cooking step may e varied, although it has been found that for most efiicient use of the digestion reagents and highest yields of desired wood fibers a caustic solution of the concentrations above-mentioned should be used in a ratio of between about 2 to 1 and 8 to l on the dry weight of wood chips, and that particularly good results are obtained using a caustic solution to wood fiber ratio of between 4 to l and 6 to l. in addition to the caustic alkali, the digestion liquor can have other treating reagents added thereto. For example, small amounts, e.g., 6.1% to 1% by weight, of surface active agents which are efiective in solutions having a pH of 9 to ll may be included in the digestion liquors in order to foster penetration of the wood chips by the cooking liquor.

It is not necessary to remove the bark from the hardwood logs or slabs before they are chipped, although de barking may be employed if desired. The chipping may be accom lished on any standard available type of chipping equipment, but a chipper is preferably chosen and operated to obtain chips having maximum uniformity in size. Chips which will pass a standard one inch mesh sieve and be retained on a standard /4 inch mesh sieve constitute a highly desirable type of chip for use in the new operations. While other size chips may be employed, it is recommended that chips be employed having an average size capable of beir screened as indicated.

After the completion of the digestion operation, the cooking liquor is withdrawn from the digestion vessel through suitable screen devices and sent to st rage tanks for reheating or re-use in subsequent digestion batches. However, the mass of digested wood chips, following this removal of the reaction liquor, containsan appreciable amount of reactive liquor in contact with the chips. If suitable storage facilities and other handling equipment are available, the drained mass of digested chips may be stored for periods of time ranging from A. hour to more than several hours. During such storage in the presence of the residual caustic solution, further digestion of the chips occurs. Hence, an after-storage procedure of this type can be com ined with the digestion operation operated at elevated temperatures and super-atmospheric pressures in a closed digestion vessel to shorten the time of batch treatment in the digester, thereby increasing the output per unit time for each digester employed at any particular pulp plant.

Following the digestion operation, it is necessary to subject the cooked wood chips to a refining operation in which defibration of the chip mass is accomplished. There are a variety of suitable types of refining or defibration equipment commercially available for this purpose. A disc type pulp refiner, e.g., a Bauer refiner, in which the digested wood is forced between rotating grinding plates spaced approximately 0.095 to 0.1 inch from each other, is found to be very suitable for this purpose.

The working of the cooked chip mass is conducted in one or more stages in suitable refiners or in suitable refining and beating equipment and for su'lhcient time to produce a pulp having a Tapp drainage time of 10 to 60 seconds. The working can be in a single stage in one or more refiners or heaters. The exact procedure used for this chip working will depend to some extent upon the particular hardwood used in the digestion step, and upon the temperature, time, pressure and concentration of the digestion liquor during the chip impregnation period. Hence, the details of the operation of this wort;- ing cannot be accurately specified for all of the varied digestion conditions, but the refining step can be satisfactorily handled by periodic testing of the drainage time of the pulp being processed and discontinuance of the refining on a particular batch of pulp when the drainage time comes within the range of 10 to 60 seconds. The fiber working preferably should also be conducted to produce a pulp having the following additional properties:

When the caustic liquor reacts upon the chips, various water-soluble salts and other by-products are termed which should be removed from the fibers before the formation of the insulating board, since the presence of such materials in the board decreases the water-resistance of the board. Consequently, it is necessary to subject the fibers, following the digestion and refining steps, to one or more washings in order to remove occluded salts, caustic, and other Water-soluble materials. It has been found that effective washing of the wood fibers can be accomplished on filtering screens Where the fiber mass is dewatered subsequent to the refining procedure as previously described. Such washing can be accomplished by spraying the fiber mass with hot water from shower heads located over the top of the filtration screens. However, it has been found that the wood fibers produced in accordance with this invention have a tendency to retain some of the caustic cooking liquor even after a water washing. Hence, it has been found important to subject the refined wood fibers to a neutralization step using some suitable acidic reagent which will effectively neutralize alkaline material. Sulfuric acid is the preferred acidic reagent for the neutralization, but other mineral acids, such as hydrochloric acid, phosphoric acid and the like, may be used, as Well :as acidic salts, including sodium bisulfate, ammonium bisuliate, and the like. This neutralization is best carried out with the Wood fibers suspended as a slurry of about 1% to about 10% fiber concentration in a suitable vessel equipped with impeller type or other adequate stirring device, suflicient acidic reagent being employed to bring the pit of the slurry to between about 5 and about 7.

Wood fiberboard may be made from a furnish formed of the caustic pulp, produced as described above, and water without the addition of any other ingredients. Thus, the fibers made by these new pulping operations are strongly cohesive when formed into webs and dried,

and create sheets or boards having high tensile strength,

. good modulus of rupture, and other desirable properties. However, the furnish used for forming the insulating board can have a variety of other materials added to it,

including sizing agents, waterproofing agents, fireproofing agents, dyes, pigments, bleaching agents, or the like.

customarily, before the furnish ispassed to the head box of a board forming machine, alum or equivalent reagent I will be added to it to adjust the pH to between about 4:5

; of solids, of which at least about 70% by weight constitute wood fibers, either prepared as described above or 'obtained' by grinding of hard or soft Woods or from 'waste paper or like waste sources, e.g.,waste kr'aft fibers. The precise proportion of the hardwood fibers prepared by the 'new digestion processes herein described toother -fibers included in the board forming furnish may be varied. The new wood fibers provided by this development, however, are characterized by a critically improved degree of freeness and a substantial lack of spring- "back when subjected to compression in wet webs. Ac

cordingly, it is advisable to use at least 25% by weight of .the total fiber content of the furnish of the new caustic hardwood pulp, and it is especially preferable that at least 50% to 100% of the total fiber content of the furnish should be the new caustic hardwood fibers.

A variety of board forming equipment is available for producing insulating board from furnish of the type just applied .to the wet mat,

the belts'at the inlet end. However, it has been found Ithat equally satisfactory results" with fewer equipment problems can be obtained using a compression roller arrangement in which a roller is used; to compress the wet fiber mat against .a traveling, supporting belt or against'another roller rotating at the same speed as the compression; roller. The degree of compression applied to the mat will depend primarilyupon the density de-- sired in'th-e final rigid insulating board. If a boa-rd hav-.

ing the lightest density is desired, e.g., about 10 pounds per cubic foot, substantially no compressive-force will be On the other hand, if insulating board of the highest obtainable density, e.g., up to about pounds per cubic foot, is desired, the board will be compressed to the maximum to reduce the wet mat thickness as it is obtained from the cylinder machine by 50% or more. As previously indicated, one of the distinct advantages of the new hardwood pulps of this development is the lack of tendency to spring-back after this compressive step so that the fiber densities imparted to the board by the compression step'will be retained there in. This isin contrast to the substantial spring-back effect noted in fiber matsjformed from neutral sulfite pulp, ground wood pulp, and the like.

After the squeezing or compressive step just described, it is difficult to remove further water from the mat by suction or pressing operations. Accordingly, in order to dry the mat it is necessary to heat it in some suitable 'fashionbecause drying at ambient temperature is too slow for any commercial board forming operation. The heating can be accomplished by subjecting the mat to infra-red, radiation, such as. from a bank of infra-red described. This includes Fourdrinier type machines and so-called'cylinder machines, e.g., a continuous Oliver cylinder machine in which a foraminous cylinder is rotated partiallysubmerged in a bath of fiber slurry with the interior of the cylinder being under substantial vacu- ,um causing water to be drawn through the foraminous cylinder while the fiber content of the. slurry is retained on the rotating cylinder forming a partially dewatered mat of fibers. a

Using the new furnish, as described, with a cylinder 7 type board forming machine, it has been found that no difiiculty is experienced in forming the pulp into board having a final dried thickness of A to /2 inch and a density of 30 to 35 pounds per cubic foot at forming speeds of 20 to 40 feet per minute. Of course, as the thickness of the board increases, the output of a board.

forming machine will decrease because the increased thickness requires an increasingly thicker fiber web to be picked up on the forming machine. For the production of a /2 inch thick board made of 100% of the new caustic hardwood fibers and possessing a density .of 30 pounds per cubic foot, it has been found that a pick-up of 1,250 to 1,450 pounds per thousand square feet is necessary for a board forming speed of 20 feet per minute.

The drainage time, e.g., 25. to 30 seconds, possessed by these new hardwood pulps, makes it possible to form V insulating board of such thickness and density at these production rates. At the same time, the new hardwood fibers may be. subjected to a maximum wet compaction and shrinkage during drying to give the insulation boards thenew high densities as reported herein.

Following formation and partial dewatering of the fiber mat on a foraminous support, the wet mat should be compressed in order to further express water therefrom and to densify the fiber content. This can be accomplished in a number of ways. For example, the mat may be passed between two traveling belts, both moving at the same speed and spaced closer together at the exit end than the inlet end, so that as the mat passes between them its thickness is reduced to the distance between ment.

lamps, by passing the mat between spaced electrodes connected to a high-frequency oscillator to heat the web by induction currents, or the like. .However, for largescale commercial operations, it has been found that T banks of long tunnel dryers equipped with internal radiant steam or electrically heated coils, and forced draft withdrawal of moisture-laden air, are most satisfactory. Preferably, the drying ovenis operated with the temperature at the inlet end of the oven lower than at the outlet end so that the web is subjected to progressively increasing temperature as it passes through the drying equip- Although the new caustic hardwood fibers of this development exhibit a substantial degree of shrinking during drying andthereby produce high densities in the final insulating boards, the drying characteristics of the fiber are excellent,.and the boards exhibit relatively low tendency to warp during the drying. It is believed that it is the shrinkage of the fibers, primarily, which causes boards utilizing such fibers to possess ahigh degree of resistance to spring-back and permits the manufacture of boards in thedensity range of 25 to 35 pounds per cubic'foot. When the fibers shrink they become more tightly intertwined and entangled and the tendency to resist spring-back is increased.

, The preceding description of the invention has been primarily confined to the description of the utilization of the instant process for the formation of insulation board, as defined herein. Howevenone of the particular advantages of the instant invention is the adaptation of the instant process for the manufacture of denser types of fiberboards, utilizing the basic insulation board formed, either in its final wet form (i.e., cold-compressed) or in its ultimate dry form; These greater density boards are produced primarily by transferring theinsnlation boards of the instant invent-ion to a hot press, wherein theboards are subjected, at least momentarily, to high pressures, in the range of approximately to 1,200 pounds per square inch, and to relatively high temperatures. For

example, in the manufacture of semi-hardboard, the wet compressed mats may be subjected to an initial pressure of approximatelyj200 pounds per square inch'and a temperature of400-450 F.,' thereby densifying the boards still further, after which the pressure is released and a spouses lower pressure is maintained for a length of time sufiicient to consolidate the mat further. In the manufacture of hardboards having a specific gravity of approximately 1.0, the wet mat, formed in the manner described herein, is inserted in a press and subjected to a pressure in the order of approximately $004,000 pounds per square inch at a press temperature of approximately 300-750 F. The pressure is released after several minutes and reduced to a lower pressure in .the vicinity of approxinately 100 pounds per square inch. The lower pressure may be maintained for approximately 20 minutes, after which time, the board is removed upon release of pressure. The removed hardboard is densified to a specific gravity of approximately 1 and is completely dry.

The above processes described for the formation of semihardboard and hardboard utilize what is known as a wet-press technique, since the mat entering the press is in a wet condition, having a fiber solids content in the order of about 25% to 65%. Where dry insulating boards are utilized, the boards may be subjected to a dry-press technique for formation into semi-hardboards and hardboards. In such technique, the dry insulating board is subjected to heat and high pressure for a short period of time. The pressure is then reduced to a much lower pressure; however, the lower pressure of the cycle need not be applied as long as the time of low pressure application of the wet-press techniques, since most of the water has been driven out from the boards by the previous drying cycle.

CONCLUSION The foregoing specification describes new processes for the preparation of wood fibers from hardwood, which fibers may be used to produce furnish that can be converted, with decided advantages in production economy, on standard fiberboard forming equipment, e.g., rigid insulating board forming equipment, into a variety of construction sheets or the like. Because the new hardwood fibers, which can be obtained from these new pulp forming methods, possess certain new and unique characteristics, it is possible to form insulating boards having density and strength characteristics, heretofore unobtainable in this class of product. Furthermore, the new hardwood fibers, as described herein, are compatible in all proportions with ground woods, waste kraft, or other wood fibers or wood materials used to form fiberboards. As a result, the new hardwood pulp may be used to form insulating boards, for example, having a range of density from below pounds per cubic foot to approximately 35 pounds per cubic foot.

The invention makes possible the novel types of formation of rigid insulation board, as defined herein, having the following properties:

0.25 to 1.0 25 to 35 Thickness in inches Density (pounds per cubic foot) Modulus of rupture.(pounds per square inch) 1000 to 1500 Mats of the new hardwood fibers may also be used in the formation of composite boards or of boards of the semi-hardboard or hardboard type. For example, a thin mat of the new hardwood pulp may be applied as a veneer to a thicker mat of ground wood fibers and roll pressed. The resulting veneer web upon roll pressing retains its densification while the ground wood mat springs back, creating a finished board with a veneer of substantially greater density than the main board section.

Simplification of waste disposal problems, and reduction of digester cooking times, digester corrosion and id heat requirements also constitute worth-while features of the new pulp and board forming operations as described While the invention has been described in rather full detail, it will be understood that these details need not be strictly adhered to and that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the subjoined claims.

What we claim is:

1. A process for the production of wood fiberboard comprising the steps of digesting hardwood chips in a 2.5 %10%, by weight, caustic solution for about A to 2 hours at about 79 F.-200 F. and at a pressure in the approximate range of from atmospheric pressure to 300 p.s.i.g., separating the digested chips from the body of the digesting liquor, refining the digested chips into a fibrous mass, neutralizing the separated fiber mass, forming a furnish comprising the refined fibers, forming a fibrous mat from said furnish by applying portions of the furnish to a foraminous support and partially dewatering the furnish on said support, suflicient furnish being applied per area of support to create a partially dewatered web having a thickness of at least about 1 2 inch, and then compressing said partially dewatered board to remove further water therefrom and to increase the density thereof.

2. A process for the production of pulp for use in the formation of wood fiberboard which comprises providing hardwood chips having an average size capable of passing a 1 inch mesh standard screen but not a Mr inch mesh standard screen, digesting the hardwood chips in an aqueous alkali metal hydroxide solution, having a concentration within the range of about 2.5% to about 10% by weight, for about to about 2 hours at about F. to about 260 F. and under a hydrostatic pressure of from about atmospheric to about 300 pounds per square inch gage, the ratio b yweight of said solution to the dry weight of said hardwood chips being between about 2 to 1 and about 8 to l, separating the resulting chips and the body of the digestion liquor from each other, refining the chips until the resultant fibrous pulp has a Tappi drainage time between about 10 and about 60 seconds, and adjusting the pH of the pulp if necessary to a value between about 5 and about 7.

3. A pulp for use in producing water-laid wood fiberboards prepared by the process defined in claim 2.

4. A process of making wood fiberboard which comprises making a furnish comprising pulp produced by the process defined in claim 2, dewatering a layer of the furnish on a foraminous support to form a wet mat comprising the hardwood fibers, cold-compressing the mat to further dewater it and to increase its density, and thereafter drying the mat.

5. A process as defined in claim 4 wherein said furnish comprises, in addition to said fibers, ground wood and sizing material.

6. A process of making wood fiberboard which comprises making a furnish comprising fibers produced by the process defined in claim 2, at least partially dewatering a layer of the furnish to form a mat comprising the hardwood fibers, and subjecting the mat to the action of a hot press under controlled conditions of time, pressure, and temperature.

7. A process as defined in claim 6 wherein said furnish comprises, in addition to said fibers, ground wood and sizing material.

8. A process of making wood fiberboard having a thickness of the order at least about A; inch which comprises making an aqueous furnish having a solids content of which solids at least about 70% by weight constitute wood fibers, at least about 25% by weight of the total fiber content being constituted of fibers produced by the pulp-producing process defined in claim 2, and producing fiberboard'of the stated order of thickness by ,subjecting said furnish to board-making filtration and a consolidation operations. r

i 9. A process as defined in claim 8, in'whic l1 the solids ,contentofsaid furnish is of the order .of about 0.5%

by the process Ideabout 25 and about 35 pounds per cubic foot and a modulus of rupture of at least about' 1,000 pounds per square-inch which comprises vimpregnating hardwood chips with an aqueous caustic alkali solution of between about 2 .5%. and about 10% concentration, by weight, a t a'tcmperature of an order between about 70 and about 200 'F., using an elevated hydrostatic pressure suflicient to effect the substantially complete impregnation of chips during the time of contact of soluv tionand chips at the elevated temperature,,refining the impregnated chipsto produce a .pulp having a Tappi drainage time of between'about 10.and about 60 second s, adjusting the pH of the resultingslurry to a value between {about 5 and about 7,' forming an aqueous V fibrous furnish, the fiber content ofwhich includes a substantial portion of the refined fibrous material, forming a mat from the furnish by a dewateringoperation, com- RICHARD D. NEVIUS, MORRIS o. WOLK,

pressing thernat to remove further water therefrom and Referenc'es Cited by the Examiner UNITED STATES PATENTS 1,709,322 7 4/29 Richter 162-400 1,857,316 5/32 Millington 1629O 2,030,626 2/36 Ellis l62l3 2,045,096 6/36 Osborne 162l00 $2,162,943 6/39 DreWSen 162 13 2,224,135 12/40 Boehm 162-313 -'2,749,24O 6/56 Ross 162l9 2,893,909 7/59 Shouvlin 16228 FOREIGN PATENTS 151,447 5/53 Australia.

OTHER REFERENCES I Paper In dustry and Paper World, September 1947, pp. 886 and 887. 7 7

DONALL H. SYLVESTER, Primary Examiner.

Examiners. 

1. A PROCESS FOR THE PRODUCTION OF WOOD FIBERBOARD COMPRISING THE STEPS OF DIGESTING HARDWOOD CHIPS IN A 2.5%-10%, BY WEIGHT, CAUSTIC SOLUTION FOR ABOUT 1/4 TO 2 HOURS AT ABOUT 70*F.-200*F. AND AT A PRESSURE IN THE APPROXIMATE RANGE OF FROM ATMOSPHERIC PRESSURE TO 300 P.S.I.G., SEPARATING THE DIGESTED CHIPS FROM THE BODY OF THE DIGESTING LIQUOR, REFINING THE DIGESTED CHIPS INTO A FIBROUS MASS, NEUTRALIZING THE SEPARATED FIBER MASS, FORMING A FURNISH COMPRISING THE REFINED FIBERS, FORMING A FIBROUS MAT FROM SAID FURNISH BY APPLYING PORTIONS OF THE FURNISH TO A FORAMINOUS SUPPORT AND PARTIALLY DEWATERING THE FURNISH ON SAID SUPPORT, SUFFICIENT FURNISH BEING APPLIED PER AREA OF SUPPORT TO CREATE A PARTIALLY DEWATERED WEB HAVING A THICKNESS OF AT LEAST ABOUT 1/2 INCH, AND THEN COMPRESSING SAID PARTIALLY DEWATERED BOARD TO REMOVE FURTHER WATER THEREFROM AND TO INCREASE THE DENSITY THEREOF. 